The effectiveness of a weapon system is a combination of lethality and precision. The tendency of modern systems to reduce lethality and trade it for precision has gone a long way in providing sophisticated weapons of high precision, able to achieve in one round the same effect that was previously achieved with tens of rounds. There are, however, limits of what can be achieved in each situation in terms of precision of delivery. Even if technological progress is pushing the technical limits further and further, achieving higher precision of delivery comes at a cost that may become prohibitive. Flight steering mechanisms, inertial sensors and target sensors are necessary ingredients for improving the precision of delivery, but they also add to the price of guided rounds. A great deal of work is devoted to find the right combination of these elements in order to satisfy the operational requirements for a minimum cost per round. In particular, different guidance principles have been devised to address the need of higher precision, while keeping the costs limited. Inertial guidance systems (with their modern GPS-aided variants), beam rider systems, command guidance, active, semi-active and passive homing guidance systems are examples of such guidance principles that are currently used for various purposes. Each of them has its own advantages and limitations. Each of them comes with a different distribution of resources between the round and the launching weapon system, and each of them has different requirements in terms of weapon system support, communications, etc. Typical for all these guidance principles is that they aim at improving the precision of each projectile separately.